The present invention relates to an improved ion implantation apparatus. More particularly, the invention concerns an ion implantation apparatus in which the net space charge of the ion beam is neutralized before impingement on the target specimen.
A typical ion implantation apparatus provides a beam of ions for impingement on a target specimen such as a wafer or chip used in the semiconductor industry. The beam of ions provides particles which impinge on the specimen and penetrate its surface to provide an implant of the particle in the specimen. One of the problems that prevails in such apparatus used to ion implant charged particles on insulated substrates such as silicon-on-sapphire (SOS) or oxide coated silicon is that on impact the ions transfer positive charges which accumulate on the insulating surface as well as on islands of metal on the insulating surface of the substrate. The positive charges can accumulate to such a magnitude as to not only seriously affect deleteriously the surface quality of the specimen material but can moreover cause a fracture of the structure of the specimen. In addition, the accumulated positive charge can also deleteriously affect the orderly impingement of further positively charged particles.
Heretofore the problem of the accumulated positive surface charge has been mitigated by neutralizing the wafer surface with electrons or by neutralizing all or parts of the ions of the ionic beam with electrons before the beam impinges the wafer surface. Several apparatus of the prior art have been designed to accomplish one or more of these purposes. For example, U.S. Pat. No. 3,980,183 to R. M. Ennis, Jr. on Sept. 23, 1975 describes an ion implantation system in which electrons are "flooded" on a target to neutralize the charge of impinging ions. In another system described in U.S. Pat. No. 4,120,700 to K. Morimoto on Oct. 17, 1978 an ion implantation apparatus has a thermionic emission source which provides electrons to an ion beam to cause clusters of neutral particles in the ion beam for impingement together with ionic particle clusters on the target.